This looks like a good 'un! Possibly the most Earthlike planet to be discovered so far outside the solar system. It sounds great. Not too hot, not too cold; there is a good chance of liquid water and there is enough gravity for an atmosphere. However, a bit heavier than Earth, so probably gravity is a bit high. Also, one side has permanent day and the other side is permanently in darkness. Still, not bad, and it's "only" 20 light years away. Now, how do we get there?

An artist's impression of Gliese 581g, which astronomers say is near Earth - relatively speaking - at 120 trillion miles. Photograph: Zina Deretsky, National Science Foundation/APAstronomers have discovered a potentially habitable planet of similar size to Earth in orbit around a nearby star.

A team of planet hunters spotted the alien world circling a red dwarf star called Gliese 581, 20 light years away.

The planet is in the "Goldilocks zone" of space around a star where surface temperatures are neither too hot nor too cold for liquid water to form.

"Our findings offer a very compelling case for a potentially habitable planet," said Steven Vogt, an astronomer at the University of California, Santa Cruz. "The fact that we were able to detect this planet so quickly and so nearby tells us that planets like this must be really common."

If confirmed, the planet would be the most Earth-like that has ever been discovered in another solar system and the first strong contender for a habitable one.

More than 400 exoplanets have been discovered by astronomers, but most are gas giants, like Jupiter, that would be inhospitable to life as we know it.

Astronomers used the Keck telescope in Hawaii to study the movement of Gliese 581 in exquisite detail and from their observations inferred the presence of a number of orbiting planets. The team report two new planets in the Astrophysical Journal, bringing the total number known to be circling the star to six.

One of the planets, named Gliese 581g, has a mass of three to four times that of Earth and takes 37 days to orbit the star. Astronomers believe it is a rocky planet with enough gravity to retain an atmosphere.

Unlike the previously discovered planets, Gliese 581g lies squarely in the region of space were life can thrive. "We had planets on both sides of the habitable zone — one too hot and one too cold — and now we have one in the middle that's just right," Vogt said.

One side of the planet is always facing the star, much as one side of the moon constantly faces Earth. This means that the far side of the planet is constantly in darkness. The most habitable region of the planet would be the line between the light and dark regions.

"Any emerging life forms would have a wide range of stable climates to choose from and to evolve around, depending on their longitude," Vogt said.

The average temperature on the planet is estimated to be between -31 to -12C, but the ground temperature would vary from blazing hot on the bright side and freezing on the dark side.

"The number of systems with potentially habitable planets is probably on the order of 10 or 20 percent, and when you multiply that by the hundreds of billions of stars in the Milky Way, that's a large number. There could be tens of billions of these systems in our galaxy," said Vogt.

With all those planets tucked in so tight to their primary I bet it looks pretty cool to watch them whiz around. Almost like an over-sized Jovian moon system that a real star system.

Life would be pretty tough to evolve there though. A tidally locked world, even in the habitable zone, would wind up with all of its water locked up on the dark side as ice. And the combination of feeble light from the star and the fact that the light to the temperate "twighlight zone" would be highly oblique (think the high arctic on Earth), so photosynthesis as we know it isn't likely.

But who knows. We can only barely imagine the conditions of such a strange environment. Anything is possible.

But I'd love to visit the Gliese 581 system and I can't wait for some of the first snapshots of it with the next generation of telescopes being built. d is also a candidate as a water world and c, while very much less likely, is not completely out of the question for harbouring life. And even then, all the way at f there could be tectonics on moons and such like we suspect could be supporting life in the Jovian and Saturnian systems!

You might want to consider something before you book your vacation. Even in the most habitable location in the system, probably the temperate band on 581g, its still going to be a dusky gloomy place to human eyes.

Oh I would go in a heart beat (and with my family it would be a lifetime/generational move) even if it were a barren wasteland. Just don't expect it to be the typical "shirt sleeves" world from Star Trek.

Yeah.And while I've always wanted people to have an awareness of the cosmos around them, I've found that so many of my friends have an attitude towards it once it gets on the news very much like the fellow in the mechanised suit in this video:http://www.youtube.com/watch?v=T5d0C2g5FwgThat, or they begin to think all the world's problems can be solved...